[Ml-yokadi] Yokadi airport code sprint

[Sébastien Renard]

Hello,

I had few hours to kill in airport during my way back from Akademy to home. 
So... here's some new stuff for the next release:
- bugs keywords are now prefixed by "_" to distinguish them from user keywords
- keywords are now displayed in t_list (VIP wish)
- you can define the default database path with YOKADI_DB shell env. (VIP wish 
again).

I'll push that once I got network connection.

a+
-- 
Sébastien